A magneto-optic disk is used as an optical disk for a large-capacity memory where the capacity of erasing and rewriting information on the disk is necessary. In the case where a magneto-optic disk is used as a data recording and reproducing apparatus, in order to define disk locations containing the data, each track is divided into a plurality of blocks called "sectors" in each of which a track No. and a sector No. providing an address separate from the data is recorded as an uneven pit. This track No. and sector No. are detected based on the intensity of reflected light if an optical pick-up is used, and may be taken out as a signal different from a magneto-optic signal detected by the rotation of the plane of polarization of the light. If a predetermined location containing data is to be detected by an optical pick-up, first, a track No. is read out and the optical pick-up is moved to a predetermined track. Then, the sector No. is read out, and the optical pick-up is moved to a predetermined sector thereby ending the operation. In this way, in driving the conventional magneto-optic disks, decoding a number code is required for decoding the track No. and sector No, and for this purpose, a circuit and microcomputer or the like on a considerably large scale are required to perform the decoding operation.
Next, a conventional configuration for accessing to a predetermined location containing the data will be described specifically.
A configuration of a common track access in a conventional magneto-optic disk is shown in FIG. 1. In this configuration, an address consisting of a track No. and a sector No. on a magneto-optic disk 1 is read by an optical pick-up 2 and the difference between a target track and the present position is determined. A control circuit 3 drives a traverse motor 4 for moving the optical pick-up 2, and when the target track is approached, an address on the disk is read again, so that an actuator 5 on the optical pick-up is driven thereby to make access to the target track by jumping several tracks. Subsequently, access is made to a predetermined sector on the target track. This access will be explained by reference to a conventional case of a magneto-optic disc shown in FIG. 2. As is well known, a magneto-optic disk is formed of a substrate and a magnetic recording layer supported thereon to store information.
FIG. 2(a) is a plan view of the disk 1. In this diagram, reference numeral 6 designates tracks. Further , a mark 7 is provided on the disk 1. When the disk 1 rotates and the optical pick-up detects a sector signal on a track 6, the mark 7 is immediately detected by use of a photo-sensor. Each time the disk makes a revolution, a pulse signal is taken out and used as an index indicating the rotational position. FIG. 2(b) shows a sector signal detected from the optical pick-up 2. (This sector signal contains a track signal indicating a track No.) Each track has a plurality of sectors, at the forward end of which a sector signal 8 is produced as an address information. The signal 9 thus forms a portion having the data recorded therein magneto-optically. From the signal produced by the mark 7, the starting position of each track is determined, and a leading sector is detected and used as a timing of reading the data.
In the aforementioned configuration, it is necessary to read an address signal such as track No. and sector No. on the disk several times in order to make access to a target track. This necessitates a circuit for reading an address signal and further a circuit for judging a track No. and controlling the access, thereby posing a problem that the circuit size to accomplish this is too large for a circuit to be added to the drive.
Also, it is necessary to form a mark on the disk in order to produce an index signal as explained above. Therefore, the preparation of a disk requires a process other than merely forming tracks. Furthermore, a photo-sensor is required in the drive for detecting a mark.